Add #![warn(rust_2018_idioms)], fix warning

src/keys.rs

@@ -76,7 +76,7 @@ impl KeyPair for Ed25519KeyPair {
 }
 
 impl Debug for Ed25519KeyPair {
-    fn fmt(&self, formatter: &mut Formatter) -> FmtResult {
+    fn fmt(&self, formatter: &mut Formatter<'_>) -> FmtResult {
         formatter
             .debug_struct("Ed25519KeyPair")
             .field("public_key", &self.public_key)

src/lib.rs

@@ -41,6 +41,7 @@
 //! signatures and hashes on an event, use the `verify_event` function. See the documentation for
 //! these respective functions for more details and full examples of use.
 
+#![warn(rust_2018_idioms)]
 #![deny(
     missing_copy_implementations,
     missing_debug_implementations,